PRESIDENTIAL ADDRESS. 415 



ments— perhaps even ' waltzings '—along fault-plaues have been proved; but is 

 there anything in Scrope's position that is really more extreme than the klip- 

 theory of the present day ? . . ^ 



Klips must be regarded as blocks cut off from the main region ot an over- 

 folded or an overthrust mass. Denudation' occurring after the forward move- 

 ment suffices to explain some cases; in others, separation seems to have taken 

 place as the moving mass fell forward. The klips of hard material embedded in 

 softer strata are thus a kind of rock-spray, hurled in advance of the breaking 

 earth-wave. C. Schmidt '" in our own time pictures the transference by gravita- 

 tion of strata from above the St. Gothard gneiss to the lowland of the Lake of 

 Uri. This lowland has become enriched, then, from a scenic point of view at 

 the expense of the unstable central range. 



In times later than those of the fathers of geology, the apostle of gravita- 

 tional movement as a cause of overfolding has undoubtedly been Edvard Reyer. 

 He has recently restated his views on the origin of mountain-ranges,^' and he 

 must be satisfied with the general concurrence that sliding is a factor to be 

 considered. He may be especially pleased with P. Termier's'- description of 

 the overriding of the Pelvoux mass by earth-waves from the east, where the 

 isoclinal folding of the strata on the west side of the chain is referred to the 

 passage of sheets across them, which have since disappeared through denudation. 

 The folding, says Termier, in his admirable prose, records the outward move- 

 ment of the sheets, just as the forms of the trees in the Proven5al plain record 

 the passage of the mistral. Termier in no wise fears to speak of the progress 

 of a ' traineur ecraseur ' during mountain-building as ' soudain et rapide comrae 

 une rupture d'equilibre, le dernier acte, longuement prepare, mais joue 

 d'e7npoiteiiient, de ce drame grandiose.' 



Rupture combined with rapid movement of the rocks need not be the last act 

 of the drama; but, the more we examine the history' of folded chains, the more 

 probable it appears as a culminating episode. The infolding and infaultmg of 

 strata, such as the Siwalik Beds in India, at the base of a rising chain may be a 

 matter of slow squeezing. We see in India how denudation has been at work 

 during the overthrusting process ; but the successive movements may none the 

 less have included rapid phases. When a fan arises in a geanticlinal by nipping 

 at the base, its destruction also may be rapid, since it is assisted by rupture 

 and falling apart of the upper portions of the folds. The original cover of our 

 present ranges has been lost by deirudation. Earth-sculpture in these regions of 

 high altitude and vehement attack has removed much of the evidence that we 

 seek. What remains, however, may lead us to feel that no part of the world in 

 historic times has experienced a mountain-building episode. 



Such relatively catastrophic stages have, indeed, not been common in the 

 long history of the earth since pre-Cambrian times. It appears that now 

 and again the ' orogenic collaipse ' ot some considerable area disturbs the balance 

 in the crust and spreads far through the upper layers like a disease. Or it may 

 be that the thermal cause of the collapse is common to the whole earth at the 

 same time, and becomes manifest in responsive regions far apart. In any case, 

 the weak places give way and the more resisting ones close in. A readjustment 

 is effected, which then endures through long geological time. 



Radioactive measurements of the length of geological periods may some day 

 enable us to determine if the major disturbances of the crust are rhythmic. 

 Present results, however, do not indicate a time-relation. The figures pro- 

 vided by W. J. SoUas,^^ and based on the thicknesses of strata, give us tlie 

 following intervals between some of the best marked foldings of the crust. The 

 unit here employed is the time represented by 1,000 feet of strata. The forma- 

 tion of the Huronian chains must be set back by an unknown amount into the 

 pre-Cambrian era, since the crumpled masses, invaded by the yoiuiger series of 



'° Bild und Baa dcr Schweizer Alpen (1907), p. 68. 



= ' Geologische Prinzipienfragen (1907), pp. 142, 147, &c. 



" ' Les Problemes de la Geologic tectonique dans la Mediterranee occidentale,' 

 Revue generale des Sciences, March 30, 1911. 



" Presidential Address, Quail. Joitrn. Ucol. Soc. London, vol. Ixv. (1909). 

 Proceedings, p. 112. 



